This invention relates to digital signal processing and has particular reference to generating a demodulator reference signal so that the information in an amplitude-modulated information signal may be recovered by demodulation. It most particularly relates to generating signals for the subsequent demodulation of a quadrature-amplitude-modulated (QAM) double-sideband-suppressed-carrier (DSSC) information signal.
Information often modulates an underlying carrier signal, thereby producing a modulated information signal. This same carrier signal is required, with a few modifications, to demodulate the modulated information signal. This required signal is called the demodulator reference signal. It must be complex, that is, it must contain separate in-phase and quadrature outputs.
A degraded version of this signal is often available with the correct frequency, but with the wrong phase and amplitude, and with a zero frequency or dc offset. It may well be known that a dc offset is present, and that the phase and amplitude are incorrect by known amounts. Repairing the damage quickly and cheaply, however, is more difficult than recognizing its existence.
The source of the modulated information signal may be (for example), the quartz angular rate sensor disclosed in the cross-referenced applications. The degraded modulator reference signal may come from the apparatus driving the source of the modulated information signal (as in the above-referenced quartz angular rate sensor), or it may come from the source of the modulated information signal itself.
The present invention is particularly well suited to the above-referenced quartz angular rate sensor. However, it is well suited to any application in which there is an amplitude-modulated information signal, and in which there is a degraded modulator reference signal which must somehow be upgraded enough to demodulate the amplitude-modulated information signal.